1. Field of the Invention
The present disclosure relates to an organic light emitting diode display having high aperture ratio, and more particularly to a high aperture ratio organic light emitting diode display having a double bank structure to prevent from forming non-filling areas when depositing an organic light emitting material using an ink filling method.
2. Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP) and the electroluminescence device (or EL).
The electroluminescence display device is categorized in the inorganic light emitting diode display device and the organic light emitting diode display device according to the luminescence material. As a self-emitting display device, the electroluminescence display device has the merits those the response speed is very fast, the brightness is very high and the view angle is large.
FIG. 1 is a diagram illustrating the structure of the organic light emitting diode. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole from the anode and the electron from the cathode are recombined at the emission layer EML. The organic light emitting diode display can represent the video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer ELM of the organic light emitting diode as shown in FIG. 1.
The organic light emitting diode display (or OLED) using the organic light emitting diode can be categorized in the passive matrix type organic light emitting diode display (or PMOLED) and the active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows the video data by controlling the current applying to the organic light emitting diode using the thin film transistor (or TFT).
FIG. 2 is the exemplary circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED). FIG. 3 is a plane view illustrating the structure of one pixel in the AMOLED. FIG. 4 is a cross sectional view along the cutting line I-I′ for illustrating the structure of the AMOLED.
With reference to FIGS. 2 and 3, the active matrix organic light emitting diode display comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLED connected to the driving thin film transistor DT.
The switching thin film transistor ST is formed where the scan line SL and the data line DL is crossing. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD.
The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OD disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLED.
With reference to FIG. 4 more detail, on the substrate SUB of the active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed.
The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the gate contact hole GH penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT. On the passivation layer PAS, an anode electrode ANO of the organic light emitting diode OLED is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT via the pixel contact hole PH formed at the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank (or ‘bank pattern’) BN is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL and VDD, for defining the light emitting area.
The top surface of the substrate SUB having the thin film transistors ST and DT, and the various lines DL, SL and VDD is not even or smoothing condition but has many stepped profiles. When the organic material, such as an organic light emitting layer, is deposited on the surface having uneven or un-smoothing surface condition, the characteristics of the organic material may be deteriorated. The bank BN is for preventing the organic material from being deteriorated at the stepped portion. To divide the emitting area having the even surface from the non-emitting area having the switching thin film transistor ST, the driving thin film transistors DT, and the various lines DL, SL and VDD, the bank BN is formed over the non-emitting area. Therefore, by the bank BN, the emitting area is defined.
The exposed portion of the anode electrode ANO by the bank BN would be the light emitting area. On the anode electrode ANO exposed from the bank BN, the organic light emitting layer OLE is formed. On the organic light emitting layer OLE, the cathode electrode CAT is formed. Here, the organic light emitting layer may be a color organic light emitting layer COLE which can emit any one color allocated at each pixel.
We will explain about the emitting area defined by the bank BN at the pixel area defined by the data line DL, the scan line SL and the driving current line VDD. The anode electrode ANO is formed as not overlapping with the data line DL, the scan line SL and the driving current line VDD.
As the organic light emitting diode display is driven by the current driving method, the anode electrode ANO may be supplied with a big current through the driving current line VDD. If the anode electrode ANO is overlapped with any line DL, SL and/or VDD, the parasitic capacitance may be formed at the passivation layer PAS inserted there-between. In the related art, the passivation layer PAS has about 3,000 Å thickness. In the liquid crystal display driven by the voltage driving method, the passivation layer of 3,000 Å thickness can prevent forming of the parasitic capacitance between the pixel electrode and any line DL and/or SL.
Further, when the passivation layer PAS includes an organic material having lower permeability, even if the pixel electrode of the liquid crystal display is overlapped with lines around the pixel electrode, there is no parasitic capacitance at the overlapped portion. Therefore, the aperture ratio can be easily ensured. However, in the case of the organic light emitting diode display having the passivation layer of 3,000 Å thickness, as the current supplied to the anode electrode ANO is very large or big, the parasitic capacitance can be formed easily between the overlapping portions when the anode electrode ANO is formed as being overlapped with any line. This may be resulted the deteriorated quality of the video data in the organic light emitting diode display.
In the related art, when the anode electrode ANO is designed, it should be apart from the current driving line VDD with a predetermined distance by considering the alignment margin in the patterning process, as shown in FIGS. 3 and 4. Also, when forming the bank BN over the anode electrode ANO, by considering the pattern processing margin, the edge of the bank BN should be located inside from the edge of the anode electrode ANO. That is, the open area defined by the bank BN would be smaller than the area of the anode electrode ANO as the rectangular shape drawn by the ‘alternated long and short dash line’ shown in FIG. 3. The open area would be designed as the emitting area.
On the surface of the substrate SUB having the bank BN, the organic light emitting layer OLE is deposited as covering the anode electrode ANO. On the organic light emitting layer OLE, a cathode electrode CAT is deposited. As a result, the area where the anode electrode ANO, the organic light emitting layer OLE and the cathode electrode CAT are overlapped at the same time would be the emitting area.
With reference to FIG. 5, we will explain about the relationship between the pixel area and the emitting area according to the related art. FIG. 5 is a plane view illustrating the lowered aperture ratio by that the organic light emitting material is not filling to the corner area of the emitting area defined by the bank, in the AMOLED according to the related art.
There are some methods for depositing the organic light emitting layer OLE including the vacuum heating depositing method or the ink-jet printing method. As explained above, when the organic light emitting layer OLE is deposited over the bank BN having rectangular shape, the organic light emitting material may not be filled at the corner portions. This area in which the organic light emitting layer OLE is not deposited is called as the Non-fill Area.
As a result, the actual emitting area is defined as the oval shape represented in the dotted line as shown in FIG. 5. That is, the emitting area is much smaller than the anode electrode ANO, especially, it is much smaller than the anode electrode ANO exposed by the bank BN. According to the related art, the aperture ratio, the ratio of the emitting area to the exposed area of the anode electrode would be lowered.